Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimable dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, is heated by means of a thermal printing head provided with a plurality of juxtaposed heat-generating resistors, so that dye is transferred from the selectively heated regions of the dye-donor element to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which has been covered with a dye layer comprising the printing dyes. Usually, an adhesive or subbing layer is provided between the support and the dye layer. Normally, the opposite side of the support is covered with a slipping layer that provides a lubricated surface against which the thermal printing head can pass without suffering abrasion. An adhesive layer may be provided between the support and the slipping layer.
A dye-image receiving element for use according to thermal dye sublimation transfer usually comprises a support, e.g. paper or a transparant film coated with a dye-image receiving layer, into which the dye can diffuse more readily. An adhesive layer may be provided between the support and the receiving layer. A releasing agent may be contained in the receiving layer or in a separate layer on top of said receiving layer to improve the releasability of the receiving element from the donor element after the dye transfer has been effected.
The dye layer can be a monochromic dye layer or it may comprise sequential repeating areas of differently coloured dyes e.g. dyes having a cyan, magenta, yellow, and optionally black-colour hue. When a dye-donor element containing three or more primary colour dye areas is used, a multicolour image can be obtained by sequentially performing the dye transfer process steps for each colour area.
Black-coloured images can be obtained by thermal dye sublimation transfer printing either by sequentially performing the dye transfer process steps for the three primary colours cyan, magenta, and yellow by means of a dye-donor element comprising sequential repeating areas of cyan, magenta, and yellow coloured dyes or by performing only one transfer step by means of a dye-donor element having a black-coloured dye layer containing a fixture of yellow, magenta, and cyan coloured image dyes. The latter of these two methods is preferred because of i.a. the ease of manufacturing the donor element containing only one dye area, less time-consuming recording with only one transfer step, and avoiding the problem of transfer in register of the respective dyes in the respective dye areas. Mixtures of yellow, magenta, and cyan dyes for forming a black-coloured dye layer of such a black-coloured dye-donor element have been described in e.g. European patent application N.degree. 92202157.1, EP 453,020, U.S. Pat. No. 4,816,435, and JP 01/136,787.
An important application of the recording of monochromic black images by thermal dye sublimation transfer is the recording on transparant film receiver of hard copies of medical diagnostic images formed by e.g. ultrasound techniques. Such a hard copy is considered to be an ecologically more acceptable and more convenient substitute for the black-and-white silver hard copy formed by development of conventional photographic silver halide film materials where processing solutions comprising silver salt residues have to be treated carefully before disposal.
In order to be a really valid substitute for conventional photographic silver halide materials the black-coloured fixture of organic dyes used in thermal dye sublimation transfer printing should behave optically as black silver.
In the medical world physicians and radiologists evaluate X-ray photographs or other images on a light box or negatoscope. These light boxes contain fluorescent lamps as light source. The spectral emission of fluorescent lamps depends on the phosphors used in the fluorescent lamp, said phosphors having peak emissions. As a result, fluorescent lamps do not show a continuous emission spectrum. Furthermore, there is no standardization in the type of fluorescent lamp used in said negatoscopes.
There is not so much a problem when viewing classical medical images composed of silver metal on the light boxes, since the spectral absorption characteristics of silver are constant over the whole visible spectrum. The hue of the silver image does not change whatever the spectral properties of the light source are, by which the image is being viewed.
However, when the black image is composed of coloured dyes, problems of hue changes may arise, because the spectral absorption characteristics of organic dyes are not constant over the whole range of the visible spectrum. A black-coloured dye mixture having a neutral look when viewed with one light source will not have a neutral look when viewed with a spectrally different light source. This phenomenon of hue change of an image when viewed with a different light source is highly unwanted, especially when medical diagnostic images have to be evaluated.
The characteristics of colour dyes that can be used for composing black-coloured dye mixtures have been further described in EP-A 453020.
Many of the dyes proposed so far for use in thermal dye sublimation transfer are not sufficient in performance since they yield inadequate transfer densities at reasonable coating coverages, or because they have inadequate spectral characteristics for substractive colour systems, or because they have a poor light-fastness.
As mentioned before, for forming a black record by thermal dye sublimation transfer the transfer currently has been performed by means of a dye-donor element having a black-coloured layer containing a mixture of yellow, magenta, and cyan coloured dyes.
However, the conventional materials are insufficient in performance in that the density of the transferred black image obtained therefrom is too low, especially When transfer has been performed onto a transparent dye-image receiving element.
To fulfil the above described requirements for black-coloured dye mixtures, it would be an advantage that the colour dyes have a high molar extinction coefficient at the absorption maximum combined with high side absorptions. As a consequence, fewer dyes and/or smaller amounts of dyes would be needed to reach higher black density values, which would also result in less overloading of the polymer matrix of the dye donor layer. SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide novel magenta and cyan heterocyclic hydrazono dyes having a high molar extincton coefficient.
It is another object of the present invention to provide novel magenta and cyan heterocyclic hydrazono dyes having a high molar extinction coefficient combined with high side absorptions.
It is another object of the present invention to provide dye donor elements, in particular magenta or cyan dye donor elements, that yield transferred dye images having high densities.
It is a further object of the present invention to provide black dye donor elements that yield transferred black images having high densities.
Further objects will become apparent from the description hereinafter.
In accordance with the present invention a dye donor element for use according to thermal dye sublimation transfer is provided, said dye donor element comprising a support having thereon a dye layer comprising at least one heterocyclic hydrazono dye, wherein said at least one heterocyclic hydrazono dye corresponds to the following general formula (I): ##STR1## wherein: z represents the atoms necessary to complete a heterocyclic ring system, a substituted heterocyclic ring system including a heterocyclic ring system carrying a fused-on aliphatic or aromatic ring system,
Q.sub.1, and Q.sub.2 (same or different) represents hydrogen or a substituent e.g. an alicyclic, aromatic, or heterocyclic ring system including such ring system in substituted form, PA1 R.sub.2 represents an alkyl group, a substituted alkyl group, a cycloalkyl group, a substituted cycloalkyl group, an aryl group, or a substituted aryl group, PA1 m is 0 or 1 and PA1 Y represents the atoms necessary to complete a heterocyclic coupler system or substituted heterocyclic coupler system.
The present invention also provides novel dyes corresponding to general formula I.
The present invention further provides a dyed receiving element comprising a dye in image-wise distribution, formed by thermal dye sublimation transfer using a dye-donor element according to the present invention.
The present invention also provides a method of forming an image by image-wise heating a dye-donor element comprising a support having thereon a dye layer comprising a binder and at least one dye corresponding to any of the above general formula I, and causing transfer of the image-wise heated dye to a receiver sheet.